Method
Our poll-based analysis began in 2000 with the Bush/Gore presidential election. In 2010, we began our Senate analysis.
Using data from the most recent poll, we determine the probability for the Democratic candidate and the Republican candidate
and use these individual race probabilities to calculate the odds for every possible combination of outcomes and calculate each party's odds of winning.
We analyze poll findings from every pollster assuming the following:

The polling date, or midpoint of a multi-day poll, is more recent than the last poll we've analyzed in that race.

The poll reports the sample size and % of the vote for each candidate.

The poll is based on "Likely Voters," preferably with "Leaners."

If the poll consists of "Registered Voters", we will use it only if there is no "Likely Voters" poll within 30 days.

If the poll is an online poll, we only use it if there is no "Likely Voters" or "Registered Voters" poll available.

Here are the current individual race probabilities, rounded to the nearest whole percent, from our poll-based Senate analysis:

Currently, Republicans hold a 51-49 edge.
This year there are 35 Senate seats which are up for election, 26 will be defended by Democrats and 9 by Republicans; 42 Republicans and 23 Democrats are not up for re-election.
Republicans need to win 9 seats to retain control of the Senate; Democrats need to win 28 seats to win control.
With 35 seats, there are 2^35, over 34 billion, possible combinations of outcomes.
From the individual race probabilities, we calculate the odds for every possible combination of outcomes and calculate each party's odds of winning.
Here is the current probability distribution for our poll-based analysis:

Our poll-based analysis has correctly indicated which party would control the Senate in 2010, 2012, 2014, and 2016.
Here is our poll-based projection of the number of Senate seats for each party:

Currently, our poll-based analysis indicates that the odds the Democrats will take control of the Senate are very slim.